Before Castle’s momentous hundredth episode airs in April, an intense episode focusing on Detective Ryan will unearth sordid secrets from his past.

“I know the fans are going to be very happy with it,” star Nathan Fillian told Entertainment Weekly. “ We figure, ‘Oh, we know this guy; we love this guy,’ and then we — as a cast of characters — find out something about him that we don’t know and he has to go back into this world that he was in for this one episode. We really get to see him in a very different light."

The episode in question, entitled “The Wild Rover,” will have Ryan go back to his days as an undercover narcotics detective, according to Zap2it. As he goes back into his undercover world, he’ll run into an ex-girlfriend who has some connection to a recent murder.

“I love it when — and this happens in life — you figure you know somebody super well and they tell you something like, ‘Oh, I went to prison once.’ ‘WHAT!?!” Fillian joked. “It may not change your opinion of somebody but you certainly see them in a different light.”

Castle’s 100th episode, “The Lives of Others,” will air on April 1. In it, novelist Richard Castle is trapped in his house after having sustained injuries during a skiing accident. When he witnesses a murder across the street, he’s met with skepticism from everyone he tells. It’s meant to be one of the series' lighter-toned episodes.

Castle airs on Monday nights on ABC at 10/9c.

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